Electrical duct heater



' Aug. 9, 1966' N. E. HAGER, JR 3,265,865

ELECTRICAL DUCT HEATER Filed Oct. 9, 1963 INV EN TOR.

a I I III l v NATHANIEL E- HAGER, JR. 5 1 e 1 1 5 United States Patent 3,265,865 ELECTRICAL DUCT HEATER Nathaniel E. Hagar, J12, Manheim Township, Lancaster County, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Filed Oct. 9, 1963, Ser. No. 314305 6 Claims. (Cl. 219-549) This invention relates generally to heaters, and more particularly to electrical duct heaters. Still more particularly, the invention relates to an electrical duct heater which can efficiently deliver heated air in an unusually short period of time from start-up.

Duct heaters in general are used because they can provide dilferent degrees of heating for the air fed into different rooms when the main air stream is at the same temperature. Duct heaters therefore allow individual temperature control in accordance with thermostat settings or individual preferences in separate rooms.

However, duct heaters in the past have used coiled resistant wires in localized areas in the interior of the heating duct. The wires do not have appreciable area, and so they must be heated to a high temperature, often to a red heat. Heating of the moving air stream has thus not been very efficient. Additionally, there has been an appreciable time lag in heating the air from ambient temperature to the desired temperature. Accordingly, it is the primary object of the present invention to present an electrical duct heater which will quickly heat a flowing fluid through the duct. It is another object of the present invention to present an eflicient electrical heater so constructed to bring about a large area of contact between the moving fluid stream to be heated and the heating element in the heater, and in which the heating element may function at a reduced temperature as compared with the usual resistance wires.

These objects are accomplished in a surprisingly straightforward and effective manner. The invention contemplates an electric duct heater having a housing and a rolled heating assembly positionedwithin the housing. The heating assembly comprises a heating element secured to a support therefor. The heating element comprises an electrically conductive metal foil having a thickness in the range of about 0.0l0.0001 inch and a width in the range of about /s2 inches. Thus the heating element is in the configuration of a tape. Electrical power connections are aflixed to the heating element in order that power may be applied. The support for the heating element comprises a plane paper backing having a sheet of corrugated paper affixed thereto. The corrugated paper forms alternate ridges and grooves. When the heating element support has the heating element aflixed thereto, and the entire assembly is rolled up, the alternate grooves and ridges formed by the corrugated paper in cooperation with the paper backing form a multitude of continuous air-conveying passages. Since the heating element is afiixed to the heating element support, the heating element will line at least a portion of the circumferences of the air passages.

The housing of the duct may be of any convenient material such as metal, paper, resin, or any other material suitable for confining the air as it travels through the duct and for containing the heating assembly. Normally the duct will be round in shape, but it may be oval, square, rectangular, or any other convenient shape.

The electrically conductive metallic foil to be used as the heating element in the present invention may be any suitable foil. Metal foils of aluminum, copper, stainless steel, and the like may be used. Copper foil having a thickness in the range of 0;0001-0.002 inch, particularly "ice 0.0008 inch, is preferred in view of its attractive cost, low heat capacity per unit area, excellent electrical properties, and easy workability. The heating element itself may be cut in the form of simple strips, preferably inch wide, connected if desired to a common bus bar, or the heating element may be in the form of a serpentine heater. Different widths of foil may be used in a heating element to achieve a graded temperature in the heating element down the length of the duct in which the element is positioned. These will be described in more detail subsequently.

The paper which serves as the plane paper backing and the corrugated paper sheet may be ordinary cellulosic paper of sufiicient thickness and strength to be rolled after corrugation without collapse of the grooves and ridges formed by the corrugated paper sheet. The thickness will generally be in the range 0.001-005 inch. The paper may be treated by after-saturation, or made by beater saturation methods, or it may contain a resin as a binder which will cure after the entire heating assembly has been rolled and placed in the duct. Normally, however, such refinements are unnecessary since the housing supports the heating assembly with sufficient rigidity for most uses and most air velocities through the duct heater. If the heater is going to be used for temperature sufficiently high in which scorching of the paper may be a problem, asbestos paper and sheets may be used. It is preferred that the paper, of whatever kind, be thin in order that the mass of the heating assembly be as low as possible; a minimum heat capacity of the assembly is generally desirable. One of the primary advantages of the duct heater of the present invention is the quick heat-up, hence low heat capacity is necessary. The size of the grooves and ridges formed by the corrugated paper sheet as it is adhesively secured to the plane paper backing sheet may vary. However, it is preferred that the grooves and ridges have a height and width within the range of about 6 /2 inch, and preferably about A inch. To some extent the proposed use of a specific duct heater will control the sizeof the grooves and ridges making up the corrugations. When the grooves and ridges are larger than about Mr inch, however, heat exchange between the heating element and the air is less efficient. The corrugated paper will be adhered in normal manner using the usual adhesives to the plane backing sheet in a manner well known to the art. The adhesive used needs simply to be able to withstand the temperatures at which the particular duct heater operates.

In the drawings:

FIG. 1 shows a simplified isometric view of a serpentine foil heating element afiixed on the top of a corrugated paper which is in turn affixed to a plane backing sheet,

FIG. 2 illustrates a simplified isometric view of a heating assembly before rolling in which the metal heating element is in strip form and is positioned beneath the corrugated paper,

FIG. 3 is a simple section of another modification of the mounting of the heating element, and

FIG. 4 shows a completed heater assembly rolled and positioned inside a housing.

Referring to FIG. 1, the metallic foil 1 is in the form of a serpentine heater in which the slit 2 cut through the foil 1 forces electrical current to travel between the power connections 3 and 4 in a serpentine path. The corrugated paper 5 is aflixed to the plane paper backing 6 to form grooves and ridges which define air-conveying passages 7. In FIG. '2, the foil 1 has been cut at the slits 2 so that the electrical current will flow between the power connections 3 and 4 in one pass straight down the length of the foil 1; the slits 2 are positioned to give varying widths of the conductive portions down the length of the foil 1. In FIG. '2, the corrugated sheet 5 has first been laminated to the conductive foil 1 and is then affixed to the backing sheet 6 in a manner which places the foil 1 underneath the corrugated paper 5. In FIG. 2, the corrugated paper 5 is broken away in order that the remainder of the heating element may be seen. The air-conveying passages are at 7.

In FIG. 3, the metal foil 1 is adhesively secured tothe plane paper sheet 5, the corrugated paper sheet 6 lying on the other side of the plane paper sheet 5.

In FIG. 4, the housing 8 contains the rolled-up heating assembly of conductive metal foil, corrugated paper, and plane paper. Interior wiring, not shown, leads from the power connections on the heating element itself to the external power plug 9 which may be conveniently positioned on the housing 8 as a means for introducing electrical current into the heater.

It will be noted that when the heating element is rolled up the alternate grooves and ridges formed by the corrugated paper produce a series of parallel air-conveying passages 10 in the final heater in addition to the airconveying passages 7 formed by the ridges and grooves in the corrugated paper 5. Approximately half of the total of these air-conveying passages 7 and 10 will be line at least in part around the circumference thereof with the foil 1. Approximately half of the passages 7 and 10 will not have any heating element, as represented by the foil 1, in contact with the air flowing therethrough. Depending on the position of the metal foil 1 with respect to the corrugated paper 5 or the plane paper 6, different air passages will be partially lined with the heating element 1. Even those air passages which are not lined with the foil 1.wil1 nevertheless supply heat to air passing therethrough by virtue of the fact that the paper used, both in making the corrugations and in the backing, is sufiiciently thin that the temperature of the paper does not vary widely from the temperature of the foil 1. Accordingly, the duct heater of the present invention will heat air very quickly once sufficient electrical power is connected to the heating element.

It will be appreciated that the length of the duct heater of the present invention may be varied to suit the particular need. A series of heating assemblies may be positioned one after the other inside a housing. When a heating assembly is rolled up, it is preferred that the foil be on the inside of the curvature to minimize the chance of tearing. Greater heating and higher temperatures of air may be achieved by using longer ducts and longer heating elements positioned therein. Greater power input will also allow greater heating to a given heater. Power input should never be so high as to scorch the paper, if flammable paper is used, or burn out the heating element. It is one of the great advantages of the present invention that the duct heater may be designed for the purpose intended by using any of a variety of variables. Quick-response hair driers in which the temperature of the heating element is kept relatively low may now easily be constructed.

The following table contains data that show the quick achievement of near steady-state conditions with the duct heater of the present invention. In all instances the input air temperature was 0 F. and the heater was at 0 F. at the time the power was turned on. There were 12 square feet of heating element area in the duct heater being used and air was passed through the duct at the rate of 58.1 cubic feet per minute. The table shows the temperature in F. of the output air from the duct heater after 30 seconds, seconds, and 3 minutes.

Table Temperature in F. after- Watts, input 30 seconds 90 seconds 3 minutes I claim:

1. An electric duct heater comprising (1) a housing and (2) a rolled heating assembly positioned within the housing, the heating assembly comprising (A) an electrically conductive metallic foil heating element having a thickness in the range of about 0.01-0.0001 inch and a width in the range of about /s--2 inches and having electrical power connections thereon, (B) a paper support secured to said heating element, said supporting comprising (a) a plane paper backing sheet and (b) a corrugated paper sheet aifixed thereto to form alternate grooves and ridges which, when said support is in a rolled configura tion, define a multitude of parallel continuous air-conveying passages approximately half of which are lined at least in part with said heating element.

2. A heater according to claim 1 wherein said metal foil comprises copper foil.

3. A heater according to claim 1 in which said metal foil comprises copper foil having a thickness of about 0.0008 inch and a width of about A inch.

4. A heater according to claim 1 in which said paper backing and said corrugated paper comprise asbestos paper.

5. A heater according to claim 1 in which said alternate grooves and ridges have dimensions in the range of /8-' /2 inch.

6. A heater acconding to claim 1 wherein said heating element comprises conductive portions having varying widths.

References Cited by the Examiner UNITED STATES PATENTS 1,318,028 10/1919 Thomson 338-217 1,881,444 10/1932 Flanzer 338-218 X 1,890,339 12/1932 Romhild. 313-341 X 2,600,485 6/1952 Cox 338-212 X 2,682,596 6/1954 Cox et al. 338-293 2,731,713 1/ 1956 Schaeifer 338-286 X 2,915,613 12/1959 Norton 219-436 2,971,073 2/1961 Eisler 3382l2 3,032,635 5/1962 Kraft 338-286 X 3,163,841 12/1964 Willctt 338308 X FOREIGN PATENTS 871,600 6/1961 Great Britain.

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner. 

1. AN ELECTRIC DUCT HEATER COMPRISING (1) A HOUSING AND (2) A ROLLER HEATING ASSEMBLY POSITIONED WITHIN THE HOUSING, THE HEATING ASSEMBLY COMPRISING (A) AN ELECTRICALLY CONDUCTIVE METALLIC FOIL HEATING ELEMENT HAVING A THICKNESS IN THE RANGE OF ABOUT 0.01-0.0001 INCH AND A WIDTH IN THE RANGE OF ABOUT 1/8-2 INCHES AND HAVING ELECTRICAL POWER CONNECTIONS THEREON, (B) A PAPER SUPPORT SECURED TO SAID HEATING ELEMENT, SAID SUPPORTING COMPRISING (A) A PLANE PAPER BACKING SHEET AND (B) A CORRUGATED P.APER SHEET AFFIXED THERETO TO FORM A ALTERNATE GROOVES AND RIDGES WHICH, WHEN SAID SUPPORT IS IN A ROLLED CONFIGURATION, DEFINE A MULTITUDE OFF PARALLEL CONTINUOUS AIR-COVEYING PASSAGES APPROXIMATELY HALF OF WHICH ARE LINED AT LEAST IN PART WITH SAIKD HEATING ELEMENT. 